Patent GB-551282-A (GB)
Title: Improvements relating to phase modulation of a carrier frequency
IPC: H03C3/18
Filed: 1941-08-15
Abstract: 551,282. Phase and frequency modulation. STANDARD TELEPHONES & CABLES, Ltd. Aug. 15, 1941, No. 10400. [Class 40 (v)] A phase-modulated wave is produced by applying a base frequency to a pulse-producing device, the phase of the pulses being varied by a. signal voltage. The pulse producer may comprise a saturated inductance coil, or a vacuum valve circuit, and when alternate positive and negative pulses are produced, those of one sign may be eliminated by a rectifier. By distorting the signal voltage, the output may be given the characteristics of a frequency-modulated wave. As shown in Fig. 1, a base frequency source P is connected through a series-tuned circlit C1, L1 to a coil L with saturable core, and operates to produce voltage pulses alternately of opposite sign at or near the moments when the current is zero, as described in Specification 477,875, [Group XXXV], and as shown in full lines in Fig. 2. The negative pulses are eliminated by the rectifier R and the positive pulses pass through condenser C3 and load resistance R3. The moment of zero current in the coil L is shifted by applying to the coil a signal voltage from a source Q through a low-pass filter LPF. This has the effect of shifting the pulses forward or backward a period proportional to the signal voltage. The Specification states that such a wave comprises a large number of harmonics of the base frequency, and that each harmonic is accompanied by a family of side frequencies which together constitute a phase-modulated wave of the harmonic frequency. The sources P and Q may be connected in series with each other and with the coil L (Figs. 6 arid 7, not shown), and the rectifier R may be in series with the resistance R3, the condenser C3 being omitted. The base frequency wave may be converted to a saw-tooth or like form before its application to the coil L (Figs. 10-15, not shown) ; and the converter may comprise a balanced pair of valves 6, Fig. 16, giving an input output characteristic 8. A non-linear resistance S, in which current increases faster than applied voltage may be inserted in the common plate circuit to assist in setting up the desired characteristic. Error in the timing of the pulses, due to hysteresis in the core and other causes, may be corrected by connecting a biasing current source 15, Fig. 22, through a choke coil 16 across the coil L. The biasing current may alternatively be supplied by rectification of current from the source P (Figs. 23 and 24, not shown) ; and it may be applied to one or more auxiliary windings on the coil L, (Figs. 17 and 21, not shown). The pulse-producer of Fig. 1 may be followed by a filter which selects an harmonic with its side bands, and these may be heterodyned with an adjacent harmonic derived from the source P. The reduced frequency thus obtained together with its side bands may be frequency multiplied before transmission (Fig. 9, not shown). In another form of the invention, vacuum tubes arranged as described in Specification 547,193 are used for producing the time-modulated pulses (Figs. 25-28, not shown). In Fig. 30, the pulses are generated by a pair of pentode valves 1, 2 with their input grids excited in push-pull by the base frequency source P, and in parallel by the signal source Q. Each screen grid is connected to the opposite input grid by condensers C1, C11 and the anodes are coupled in push-pull to the output circuit. If valve 1 is conducting and its input voltage is falling, the screen current falls and screen voltage rises. This rise is transmitted through condenser C1 to the input grid of the valve 2, and the screen voltage of that valve falls, acting through the condenser C11 to accentuate the fall at the input grid of the valve 1. Sharp pulses are thus produced in the output circuit, the timing of which is dependent on the signal voltage Q. Negative pulses may be suppressed by a rectifier 15. Specification 472,352 also is referred to.